Resin materials used in forming or molding of parts used in a fabrication process of semiconductors such as IC and LSI and mount parts thereof, parts used in a fabrication process of magnetic heads and hard disk drives and mount parts thereof, parts used in a fabrication process of liquid display devices and mount parts thereof, or the like are required to have excellent mechanical properties, heat resistance, chemical resistance and dimensional stability.
Therefore, thermoplastic resins excellent in heat resistance, for example, poly(ether ether ketone), poly(ether imide), polysulfone, poly(ether sulfone) and poly(phenylene sulfide), are used as resin materials in this technical field.
With the development of high-density pitch in electronic devices, however, an electronic device tends to be charged by the influence of frictional electrification of a resin part used therein when the resin part is formed by a resin material having a surface resistivity exceeding 1013 Ω/□. The electronic device, in which static electricity has been accumulated by charging, may be damaged by discharge of the static electricity or electrostatically absorb dust suspended in the air. In a resin part formed by a resin material having a surface resistivity lower than 105 Ω/□ on the other hand, the moving speed of electric charge in the resin part is too high, so that a heavy current or high voltage generated upon discharge of static electricity gives an electronic device a fault.
From the viewpoints of protecting the electronic device from the fault by the static electricity and retaining high cleanness without attracting dust, the resin parts used in these technical fields are required to control their surface resistivities within a range of 105 to 1013 Ω/□ that is a semiconductive region. Thus, it has heretofore been proposed to use a resin material with an antistatic agent or a conductive filler incorporated therein to obtain a formed or molded product having a surface resistivity in the semiconductive region.
According to the method of incorporating the antistatic agent into the resin material, however, the antistatic agent present on the surface of the resulting molded or formed product is easily removed by washing or friction to lose its antistatic effect. When the amount of the antistatic agent incorporated is increased in order for the antistatic agent to easily bleed from the interior of the molded or formed product to the surface thereof, the antistatic effect can be sustained to some extent. However, dust sticks on the surface of the molded or formed product due to the antistatic agent bled, and the electronic device and an environment are contaminated by exudation and volatilization of the antistatic agent. In addition, when the antistatic agent is used in a great amount, the heat resistance of the resulting molded or formed product is deteriorated.
According to the method of incorporating the conductive filler such as conductive carbon black into the resin material, the electric resistivity of the resulting molded or formed product greatly varies with a little difference of the amount of the conductive filler incorporated or a little change of molding or forming conditions because the electric conductivity of the resin material is widely different from that of the conductive filler. Therefore, it is extremely difficult to strictly control the surface resistivity to a desired value within a range of 105 to 1013 Ω/□ by the method of simply incorporating the conductive filler. In addition, according to the method of simply incorporating the conductive filler, a wide scatter of the surface resistivity of the molded or formed product with the locality is liable to occur.
In order to solve the above-described problems, the present applicant proposed a resin composition with a carbon precursor and a conductive filler incorporated in combination into a thermoplastic resin, and molded products such as an IC socket molded with this resin composition (Japanese Patent Application Laid-Open No. 2002-531660 (through PCT route), and International Publication No. WO02/082592). When the molding is conducted with the resin composition comprising such respective components in specific proportions, a molded product with the surface resistivity or volume resistivity thereof strictly controlled within a limited desired range of the semiconductive region can be obtained. These documents disclose Examples, in which the resin composition was injection-molded to produce molded products such as IC sockets.
The above-described resin parts are generally molded by injection molding. According to the injection molding, molded products such as resin parts having a desired shape can be mass-produced. However, resin parts used in an electric and electronic field or the like are required to have high dimensional accuracy, and so a mold for injection molding is naturally required to have high dimensional accuracy.
In addition, since the molded product often deforms due to shrinkage and/or residual stress after the injection molding, the form of the mold for injection molding must be precisely controlled according to the shape of the molded product and properties of the resin material. Therefore, the mold for injection molding generally takes a long time to produce it, and so the production cost thereof is compelled to be expensive. Since fraction defective upon actual injection molding is also high, the cost of products is often increased. In addition, it is difficult to mold a molded product having a great thickness by the injection molding.
On the other hand, it is known to extrude a resin material to produce a stock shape for machining, such as a plate, round bar, pipe or special shape, and subject this stock shape to machining such as cutting, drilling or shearing to form a part of a predetermined shape. The method of machining the stock shape has such merits that parts produced in small quantity can be economically produced compared with the injection molding, the method can cope with frequent changes of part specification, parts high in dimensional accuracy are obtained, and parts having a shape unsuitable for the injection molding or a great thickness can be produced.
However, it is not that any resin material or extruded product is suitable for a stock shape for machining. The stock shape for machining is required to have various properties, for example, (I) to be thick-wall and excellent in machinability, (II) to be low in residual stress, (III) not to be heated in excess by frictional heat generated upon machining to cause neither deformation nor discoloration, and (IV) to be able to be machined with high accuracy to scarcely produce burr upon, for example, drilling.
Most of processing methods used in metallic materials are utilized in machining of polymeric stock shapes as it is. Even in extruded products, those having a thin wall and great flexibility are unsuitable for machining such as cutting. An extruded product too great in residual stress upon extrusion tends to deform upon or after machining, and so it is difficult to obtain a secondarily formed product having high dimensional accuracy.
In order to machine, in particular, a polymeric stock shape into a resin part used in a field of electric and electronic equipment parts, display equipment parts, or the like, it is indispensable that holes of a precise form can be formed by drilling, in addition to that cutting and shearing can be conducted with high accuracy.
An IC socket is used for inspection by a burn-in test in, for example, a semiconductor fabrication process. A great number of contact probes are inserted into a body of the IC socket. In order to form such an IC socket body by machining of a polymeric stock shape, a great number of pin-inserting holes must be formed by drilling. When burr is produced around an opening of each pin-inserting hole, deburring is required, and so operating efficiency is impaired. When a formed product with burr is used, a pin-inserting operation becomes difficult, and an adverse influence on electric and electronic equipments or the like by contamination of an operating environment by separation of burr and attachment of the burr separated is easy to occur.
According to the results of an investigation by the present inventors, it has been found that a formed product obtained by extrusion of a resin composition with conductive carbon black incorporated into a thermoplastic resin is marked in occurrence of burr upon drilling in addition to unstable surface resistivity. The reason why injection molding has heretofore been mainly adopted upon production of a resin part using a resin composition with a conductive filler or the like incorporated into a thermoplastic resin is not only that the injection molding is suitable for mass production of the resin part, but also that an extruded product composed of such a resin composition is considered to be unsuitable for machining.
In fact, the above-described 2 documents that disclose the resin composition with a carbon precursor and a conductive filler incorporated in combination into a thermoplastic resin, and the molded products molded with the resin composition only disclose Examples, in which molded products such as IC sockets were molded by injection molding. These documents also touch on extrusion. However, thin-wall formed particles such as films, sheets and tubes that are unsuitable for cutting and the like are only exemplified. These documents neither teach nor suggest anything about extruded products that are suitable for machining, thick-wall, low in residual stress and excellent in suitability for machining.